1. Field of the Invention
The present invention relates generally to an engine-speed control system for an internal combustion engine and more specifically to an engine idling speed control system whereby the amount of intake air by-passing a throttle valve is adjusted on the basis of the difference between actual idling speed and optimum idling speed.
2. Description of the Prior Art
There is well-known an engine idling speed control system for an internal combustion engine by which the amount of intake air by-passing a throttle valve is feedback-controlled in response to a signal indicative of the difference between actual engine idling speed and optimum engine idling speed. In such a prior-art engine idling speed control system for an internal combustion engine, since the amount of intake air by-passing the throttle valve is adjusted on the basis of only the difference between actual speed and optimum speed, irrespective of the air-to-fuel ratio of the mixture, the engine idling speed controlled by the system fluctuates according to the air-to-fuel ratio. In greater detail, in the case where the mixture is rich, if the amount of intake air is increased by a predetermined guantity, engine idling speed will be increased excessively; on the other hand, in the case where the mixture is lean, even if the same amount of intake air is increased by the same quantity, engine idling speed will be increased only a little and in some cases may decrease instead. Similarly, in the case where the mixture is rich, if the amount of intake air is decreased by a certain quantity, engine idling speed will be decreased excessively; on the other hand; in the case where the mixture is lean, even if the amount of intake air is decreased by the same quantity, engine idling speed will be decreased only a little.
That is to say, in the prior-art engine idling speed control system for an internal combustion engine, since the engine idling speed is controlled irrespective of the air-to-fuel ratio and therefore the difference between actual engine idling speed and optimum engine idling speed is relatively great, there exists a problem in that engine idling speed varies momentarily and results in engine speed hunting; that is, the control stability of engine idling speed is not satisfactory.